The invention provides for methods for removing contaminants selected from the group consisting of nitrogen oxides, sulfur oxides, particulates, heavy metals and acid gases from gas streams. More particularly, the invention provides for the removal of contaminants from gas streams arising from engines and other combustion devices on board ships and on land where seawater is used for scrubbing of the combustion gas streams.
The invention can also treat exhaust gas streams from chemical, metallurgical, partial and full combustion processes by the removal of contaminants such as nitrogen oxide, sulfur oxides, hydrochloric acid and particulate materials when co-scrubbing of nitrogen oxides would generate nitrates that are objectionable by-products.
The nitrogen oxides captured are separated from the other contaminants in the gas stream. This produces a smaller quantity of nitrate-containing liquid stream that needs to be processed before it can be discharged or re-used.
Combustion and chemical processes generally result in gas streams containing contaminants that need to be cleaned up before the gas stream is exhausted to the atmosphere.
Large ocean freighters, ferries, ocean liners and navy vessels use low cost hydrocarbon fuels that contain sulphur, chlorine, nitrogen and metal compounds in hydrocarbons which results in exhaust gases with contaminants such as acid gases, particulate matter and heavy metals. As per new legislative mandates, these large emitters need to clean up the flue gas stream before exhausting it to the atmosphere. Amongst many technologies and devices, aqueous scrubbing using seawater is one of the more widely used techniques to remove acid gases such as sulfur oxides, chlorine, hydrochloric acid, etc., particulates and other contaminants. Seawater is mildly alkaline and generally used in once through mode to neutralize the acidic components present in the seawater to form salts that are permitted to be discharged back to the ocean in an environmentally safe manner.
Many industrial installations using wet scrubbers such as fluid catalytic cracker regenerators and electrical generation along the sea coast use seawater for scrubbing in a once through or recycle mode.
In combustion processes, in addition to sulfur oxides, hydrochloric acid, chlorine and other acid gases, nitrogen oxides are also formed due to a number of reasons such as high flame temperature (thermal NOX), nitrogenous compounds present in the fuel (fuel NOX) or nitrogenous content of material subjected to combustion temperatures such as incineration of waste.
Nitrogen oxides formed at temperatures above 1300° F. (˜704° C.) are mainly in the form of nitric oxide, NO. Nitric oxide is the major component of nitrogen oxides in combustion processes. Nitric oxide is almost insoluble in water so aqueous scrubbing removes negligible amounts of nitric oxide from nitrogen oxide streams. Coal, solid fuels, heavy oils and other carbon feed stocks when combusted form exhaust gas streams that contain particulate matter and other objectionable contaminants such as heavy metals like mercury which may or may not be effectively scrubbed by aqueous scrubbing operations.
Amongst all absorption processes, ozone-based processes as described in U.S. Pat. Nos. 6,162,409; 5,206,002; and 7,303,735 provide multi-pollutant removal approaches and have been implemented on flue gas arising from gas and coal fired boilers for removal of multiple pollutants including nitrogen oxides, sulfur oxides, particulates, etc. Ozone-based processes are also industrially practiced in lowering emissions in metal pickling processes, fluidized catalytic cracker (FCC) regenerators, and metal recovery furnaces.
The method as disclosed in U.S. Pat. Nos. 6,162,409; 5,206,002; 6,649,132 and 7,303,735 uses the chemistry of nitrogen oxide reaction with ozone to form higher oxides of nitrogen, especially the pentavalent form or higher. These oxides are very soluble in water and are easily removed by wet scrubbing. The stoichiometric amount of ozone required to convert one mole of nitrogen oxides in the form of NO to the pentavalent form is about 1.5 moles of ozone. This number is reduced to 0.5 moles of ozone if the nitrogen oxides are in the form of NO2.
Although the methods described in these patents are effective at achieving low levels of nitrogen oxides emissions in the treated gas stream, they generate nitrate/nitric acid in the scrubber purge. The nitrate/nitric acid needs to be treated and disposed of in an environmentally safe way or must be utilized in making a useful by-product. This all adds to the expense of treating for the nitrogen oxides.
When seawater is used as a scrubbing medium, it is used in a once through mode due to its limited alkalinity. This generates a large quantity of liquid discharge from the wet scrubber. When ozone is added for nitrogen oxides removal, the purge stream will contain nitrates which will require treatment before discharge back to the sea.
The invention is able to overcome the problems that earlier methods have experienced. Contamination of scrubber purge with nitrate is mitigated so that a large quantity of seawater can be used in a once through scrubbing medium that can be safely discharged without additional treatments. A separate scrubber for nitrogen oxides removal is also not necessary thereby minimizing capital investment in retrofitting acid gas (sulfur oxides, hydrochloric acid, etc.) or particulate scrubbing equipment with nitrogen oxides control.